The present invention relates to a deflection yoke used for a color cathode-ray tube, and particularly to a deflection yoke capable of correcting a vertical pincushion distortion with a simple structure.
As for an in-line type color cathode-ray tube (CRT) with three color electron guns arranged in the transverse direction, which is one of the shadow mask type CRTs, there have been proposed various measures for correcting three electron beams in such a manner that the electron beams having passed through apertures of a shadow mask accurately land on centers of respective phosphors.
The beam landing correction has been performed by a deflection yoke system, typically, a self-convergence and pincushion free deflection yoke system.
Flattening of the screen of an inline color CRT gives rise to an inconvenience associated a vertical pincushion distortion. To cope with such an inconvenience, the related art beam landing correction method mentioned above is required to employ a number of various kinds of field control plates and magnets. This causes a problem that the image quality is degraded due to local distortion or the like and the manufacturing cost is raised.
A deflection yoke system intended to solve the above problem has been known, for example, from japane e Patent Laid-open No. Hei 5-252525. This deflection yoke system includes front field control plates arranged on the cone portion side of vertical deflection coils at positions along diagonal lines of the screen of the CRT, a front cross arm for correcting a vertical pincushion distortion of an-image, and a circuit for correcting misconvergence in the vertical direction of an image by correcting a horizontal deflection current in synchronization with a vertical deflection current.
In this deflection yoke system, the vertical deflection coils are arranged at four locations on the cone portion side.
To correct the vertical pincushion distortion, there has been also used another method in which a current, obtained by modulating a parabolic current with horizontal cycles on the basis of vertical cycles, is allowed to flow in vertical deflection coils of a deflection yoke.
As described above, a color cathode-ray tube is required to prevent a degradation in image quality due to the vertical pincushion distortion, and as one of various measures, there has been used a method of allowing a current, obtained by modulating a parabolic current with horizontal cycles on the basis of vertical cycles, to flow in vertical deflection coils.
This method is advantageous in that it can be relatively simply realized; however, since the vertical deflection coils of the deflection yoke must be driven at a low frequency, the number of turns of the vertical deflection coils becomes large, to thereby increase the impedance of the vertical deflection coils.
As a result, in the deflection yoke, a high voltage must be applied to the vertical deflection coils for allowing a specific current to flow in the vertical deflection coils. This gives rise to a problem that the circuit configuration is complicated and the manufacturing cost is raised.